1. Field of the Invention
The invention relates to the making of fast crystallizing polyalkyleneterephthalate resin compositions useful in molding applications. More particularly, this invention relates to the preparation of fast crystallizing polyalkyleneterephthalate molding compounds containing a complex of NaSCN (sodium thiocyanate) and an organic compound, such as a polymeric compound, containing an ethyleneoxy repeating unit together with appropriate reinforcing agents, fillers and/or additives.
2. State of the Prior Art
Polyalkyleneterephthalates have acquired considerable attention as starting materials for the preparation of film, fiber and shaped articles due to their outstanding mechanical and thermal properties. However, polyethyleneterephthalate is known to have a very slow crystallization rate at temperatures below 130.degree. C. so that when low molding temperatures and short residence times are employed the molded article has low crystallinity and poor surface smoothness. The resultant necessity to use high mold temperatures and long residence times has, for a long time, discouraged the use of polyethyleneterephthalates as a molding resin despite its high mechanical and thermal properties.
Polypropyleneterephthalate and polybutyleneterephthalate both have fast crystallization rates, but their crystallization rates and degree of crystallinity still need to be improved, especially for molding very thin parts at low temperatures.
The effectiveness of a crystallization promoter can be studied by measuring the melt and cold crystallization rates of the polyalkyleneterephthalate resin compositions. A much more convenient way is to measure the melt and cold crystallization temperatures of the compositions. Conventionally, the melt crystallization temperature can be determined by following the heat capacity change of the melt in a calorimeter. The maximum of the exotherm during the slow cooling of the melt is taken as the melt crystallization temperature (Tmc). The maximum of the exotherm during a more rapid heating of an amorphous polyalkyleneterephthalate is taken as the cold crystallizing temperature (Tcc). To obtain the amorphous or glassy polyalkyleneterephthalate, the resin has first to be completely melted and then quickly quenched, for instance by dropping the melt into dry ice. It should be noted that both the obtained melt and cold crystallization temperatures depend not only on the history (melt temperatures, annealing, etc.) of the specimen, but also on the heating and cooling rate of the measurement. Thus, for studying the effectiveness of a crystallization promoter, these conditions have to be fixed for purposes of direct comparison. These temperatures are obtainable using a differential scanning calorimeter.
Due to the low heat conductivity of the polymer molding compounds, the inside portion of the resin usually experiences during molding a slower cooling rate than the surface of the molded article. The study of the melt crystallization thus can provide information relating to inside portions whereas the study of the cold crystallization can provide information relating to the surface of the molded article. A desirable crystallization promoter will substantially increase the melt crystallization temperature and decrease the cold crystallization temperature of polyalkyleneterephthalate resin compositions.
In 1968, the British Pat. No. 1,104,089 proposed the use of a finely divided inorganic solid, such as talc, as a crystallization promoter for polyethyleneterephthalate molding compounds. For purposes of comparison and study, the crystallization promoter described herein is tested against talc.